This invention relates to processes for conversion of hydrocarbons. More specifically, the invention relates to processes for upgrading reformate, in conjunction with naphtha reforming.
The present invention relates to processes for upgrading the reformate product of an effluent stream from a reforming unit. Specifically, the present invention relates to processes of upgrading for increasing the benzene, xylene and C5- content of a reformate product.
Benzene is a highly valuable product for use as a chemical raw material. Xylene and, in particular, para-xylene, is a valuable chemical feedstock which can be separated for use in the synthesis of polyesters from mixed xylenes by fractional crystallization, selective adsorption, or membrane separation.
The presence of benzene and xylene in petroleum and other naphthas has long been recognized and many techniques have been described for recovery of the single ring aromatics from their admixture with aliphatic hydrocarbons of like boiling point. Early practice favored extraction with selective solvents to separate aromatics from aliphatics. The aromatic concentrate so derived was readily distilled to yield benzene and toluene of desired purity. The four C8 aromatics, ethylbenzene (EB) and the three xylene isomers posed a more difficult problem because of their boiling points which are within the narrow range of 277.1xc2x0 F. for EB to 292xc2x0 F. for orthoxylene (o-xylene). The great differences in freezing point provided a basis for separating C8 components by fractional crystallization. Extensive facilities were installed for recovery of para-xylene (p-xylene) in this manner,after which the C8 aromatics mixture lean in p-xylene could be subjected to catalytic isomerization to generate additional desired p-xylene from o-xylene and meta-xylene (m-xylene) and the product enriched in p-xylene recycled to the fractional crystallization stage. This left EB unchanged to accumulate in the isomerizer/crystallizer xe2x80x9cloopxe2x80x9d unless steps could be taken to remove it.
One approach to that problem of EB build-up was the Octafining process which isomerizes xylenes under hydrogen pressure in the presence of platinum on silica-alumina catalyst. Under Octafining conditions, EB is hydrogenated to ethylcyclohexane, isomerized to dimethylcyclohexane and dehydrogenated to xylene. Concurrent conversion of xylene to undesired by-products is such as to result in net loss of xylene despite the conversion of EB to xylene. That history of Octafining is set out in more detail by U.S. Pat. No. 3,856,872, which is incorporated herein by reference, and which discloses isomerization in the presence of a catalyst in which the acidic component is a zeolite such as ZSM-5.
As commercial use of xylene has increased, isomerization of the other C8 aromatics to produce equilibrium mixtures of xylenes, and thus increase the yields of the desired xylenes, has become increasingly important. At present, several xylene isomerization processes are available and in commercial use. For example, U.S. Pat. Nos. 4,163,028 and 4,236,996, both of which are issued to Tabak et al. and incorporated herein by reference thereto, disclose processes for isomerizing xylene and converting ethylbenzene to benzene at high temperatures in the presence of a ZSM-5 catalyst. The processes provide for the conversion of ethylbenzene to benzene without the loss of xylenes by disproportionation.
One process which as been employed for the conversion of paraffins and/or olefins to aromatics is M-2 Forming, which is described in U.S. Pat. Nos. 3,760,024 and 3,756,942 to Cattanach, U.S. Pat. No. 3,845,150 to Yan et al., U.S. Pat. No. 4,090,949 to Owen et al. These patents are also incorporated herein by reference in their entirety. M-2 forming is concerned with upgrading relatively poor quality olefinic gasoline, for example, by conversion thereof in the presence of hydrogen and/or carbon hydrogen contributing fragments and an acid function catalyst comprising a crystalline zeolite of selected pore characteristics, such as ZSM-5.
U.S. Pat. Nos. 4,851,604; 5,365,003;5455,213 and 5,498,822 discloses the MTPX process, which is a method for converting toluene to para-xylene. Shape selective hydrocarbon conversions are effected by modifying a catalytic molecular sieve, such as ZSM-5, which has been selectivated by contact with a silicon selectivating agent selected from the group consisting of silicones and silicone polymers. The silicon-containing selectivating agent is present in an organic carrier. The molecular sieve is subsequently calcined. The conversion conditions comprise a temperature of from about 100xc2x0 C. to about 760xc2x0 C., a pressure of from about 0.1 atmosphere to about 200 atmospheres, a weight hourly space velocity of from about 0.08 to about 2000, and a hydrogen/hydrocarbon mole ratio of from about 0 to about 100.
U.S. Pat. No. 5,406,016 discloses a process for simultaneously converting benzenes to predominantly methylbenzenes and simultaneously reducing the concentration of C10+ alkyl aromatics in a naphtha boiling range refinery process stream. The stream is contacted at a temperature in the range of about 250xc2x0 C. to 450xc2x0 C., and a pressure of about 400 to 2500 psig, with a 12-ring zeolitic material such as USY, faujasites and zeolite beta. The zeolite is loaded with a metal having a hydrogenation function, such as Re.
A process for transalkylation of alkylaromatic hydrocarbons is disclosed in EP 816311A. This process exhibits a percentage conversion of ethyltoluene higher than 50 wt %. The hydrocarbons are contacted with a catalyst composed of mordenite (100 pbw), inorganic oxide and/or clay (25-150 pbw), and at least one metal component selected from rhenium, platinum and nickel. Xylenes are a preferred product.
The effluent stream from a reformer contains chemicals which may be converted to more valuable products, such as benzenes and xylenes. For example, reformate typically contains significant amounts of n-paraffins which have low octane value and toluene which can be disproportionated to benzene and xylenes. Because the reformate is already at elevated temperature, the stream is highly suitable for further conversion over a catalyst, preferably a shape selective zeolite. Specifically, the desired reactions which can be achieved using a catalyst comprising a shape-selective zeolite catalyst are: conversion of n-alkanes with low conversion of isoalkanes, dealkylation of alkylated aromatics (e.g. ethylbenzene, p-ethyltoluene, propylbenzenes, etc.), toluene disproportionation to xylenes and benzene, and selective toluene disproportionation to benzene and p-xylene. Catalysts suitable for toluene disproportionation may comprise zeolites, or non-zeolitic materials, although shape selective zeolites are preferred. The process schemes described herein have the most potential value at refineries where chemicals are highly valued.
Most previous processes employing reformate upgrading have used as feed reformate that had been fractionated prior to entering the upgrading reactor.
U.S. Pat. No. 5,865,986 discloses a process for upgrading a petroleum naphtha fraction. The naphtha is subjected to reforming and the reformate is cascaded to a benzene and toluene synthesis zone over a benzene and toluene synthesis catalyst comprising a molecular sieve of low acid activity. The preferred molecular sieve is steamed ZSM-5. The benzene and toluene synthesis zone is operated under conditions compatible with the conditions of the reformer such as pressures of above about 50 psig (446 kPa) and temperatures above about 800xc2x0 F. (427xc2x0 C.). In one aspect of the invention, the benzene and toluene synthesis catalyst includes a metal hydrogenation component such as cobalt, nickel, platinum or palladium. In one mode of operation, the benzene and toluene synthesis catalyst replaces at least a portion of the catalyst of the reformer. The process produces a product containing an increased proportion of benzene and toluene, and a reduced proportion of C8 aromatics, particularly ethylbenzenes, as compared to the reformate.
The instant invention differs from that disclosed in ""986 in that the latter primarily accomplishes dealkylation of heavy (C9+) aromatics. ""986 identified Pd-impregnated low-activity ZSM-5 (approximately 10 alpha) as the preferred catalyst for obtaining higher BTX yields. For the instant invention, the emphasis is on toluene disproportionation rather than dealkylation of heavy aromatics. Zeolite catalysts that exhibit toluene disproportionation activity are the most suitable.
This application discloses an integrated process for reformate upgrading. Such a process enables production of a high value product slate, at potentially low cost since the existing reformate stream already contains hydrogen and is at elevated temperature. The step of reforming is incorporated along with reaction/diffusion with a zeolite. In the instant invention, reformate upgrading occurs prior to fractionation. Furthermore, the upgrading catalyst performs toluene disproportionation, ethyl benzene dealkylation and/or cracking of paraffins.